1. Field of the Invention
The present invention relates to a motor drive system and a control method for AC motors.
2. Description of the Related Art
As a control method without using a rotation speed sensor or a position sensor of an AC motor, a motor drive system has been well known which detects phase currents of a motor as disclosed in JP-A-2001-251889, for example.
As a method without using a current sensor, there has been proposed, for example in JP-A-2-197295, a current reproducing method which detects a DC current of the inverter to drive a motor and reproduces an AC current of the motor from an instantaneous DC-current value detected and also from the switching states of the inverter. In this method, gate pulses to drive the inverter are used, a motor current which instantaneously appears in the DC current of the inverter is sampled and held, and thus, a motor current is indirectly detected.
With the current reproducing method, however, which reproduces a motor current on the basis of the inverter's input DC current and a gate pulse signal, it is difficult to capture a motor current component when the gate pulse is extremely short. Above all else, the higher the average switching frequency (carrier frequency) is set, the shorter the gate pulse becomes, making it more difficult to reproduce a current. As an antidote for this, if measures are taken to set a lower carrier frequency of the inverter, this increases higher harmonics of the current, which causes lower efficiency and results in occurrence of electromagnetic noise. Moreover, it becomes necessary to perform current sampling a minimum of twice in a carrier period of the inverter, and provide a special circuit. And on top of that, it is an imperative requirement to provide two analog input terminals to realize a one-chip microcomputer, and it is also necessary to mount two sets of AD converters or provide a high-speed AD converter to read the current continuously.
Therefore, the object of the present invention is to provide a motor drive system and a control method for the drive system that realize a high-performance motor drive at high carrier frequencies in a simple control structure.